SOUNDINGS

While the FOUS60, FOUS14 and graphic model data output provides a great deal of useful information when forecasting for freezing drizzle, actual raobs and forecast soundings can provide a picture of the trends in temperature, saturation, and cloud depths.

An actual Minneapolis RAOB from 12Z January 27th, 2002.  Not much going on here in terms of freezing drizzle.  There is a large area of dry air above the surface, but the lower level cloud deck is just starting to develop and is very, very shallow.

An actual Minneapolis RAOB from 00Z January 28th, 2002.  The lower level cloud deck has formed and temperatures in the cloud deck are exactly between 0C and -10C.  Drier air aloft is still in place as well.

An actual Minneapolis RAOB from 12Z on January 28th, 2002.  The lower level cloud deck is even thicker and cloud temperatures remain between 0C and -10C.  Dry air aloft is still in place.  Check out the directional shear in the lowest levels as well.  This sounding strongly indicates a chance of freezing drizzle.

An actual Minneapolis RAOB from 00Z January 29th, 2002.  This is another sounding that indicates a chance of freezing drizzle.  While temperatures in the lower level cloud deck are dropping, they are still mostly at -10C or above.  Again, check out the directional shear at the lowest levels.

An actual Minneapolis RAOB for 12Z January 29th, 2002.  This sounding now indicates that any  falling precipitation would likely be in the form of snow instead of freezing drizzle.  The entire sounding becoming moist, thus losing the dry air above the lowest cloud deck.  Temperatures within the lower level cloud deck are now almost all below -10C, which is generally too cold to support freezing drizzle and results in snow.